1. Field of the Invention
The current invention relates generally to the field of ink jet printers and plotters and more specifically to those printers and plotters with multiple print heads.
2. Description of the Related Art
Ink jet printers and plotters fall within a class of non-contact type printing where an image is formed on the surface of the medium by depositing droplets of ink from nozzles onto the print medium. The ink droplets are formed by heating a small portion of ink and selectively expelling it from a nozzle located on the face of a printing element or print head. Each print element or print head will have numerous nozzles from which ink droplets are expelled. A typical print head will have a column of more than 100 nozzles.
The vertical height of a strip of ink droplets a printer deposits in a single pass of the print head over the media is referred to as the swath height of the printer. The time it takes to print a sheet of paper of a fixed dimension will be the number of passes the print head would have to make to cover the vertical length of the paper, which depends on the swath height. Printing processes may also involve several passes over the same swath height for various reasons not directly related to this invention, but the print time for a sheet in these processes will also be dependent upon the swath height.
In some printers, the swath height of one particular color of ink is increased by mounting more than one print head for that color on the printer carriage. By mounting another print head in a manner such that its nozzles are vertically offset from the nozzle locations of the first print head, the swath height of the printer for that color is effectively increased, thereby allowing fewer passes and faster print times for a given size paper. Generally, the two print heads are arranged so that the upper few rows of nozzles on the lower print head overlap the lower few nozzles of the upper print head. The region of swath height that can be covered by nozzles from both of the print heads is referred to as the overlap region of the print heads. An example of an overlap region could be a ten nozzle region of two overlapping print heads each having 100 nozzles so that the effective swath height of the two print heads together would be 190 nozzles.
Due to manufacturing constraints the relative position of the second print head cannot be guaranteed to be exact with respect to the first print head. If the nozzles of the second print head are not exactly aligned with those of the first print head, image quality will be diminished. If the misalignment is significant enough, visible discontinuities may develop. Common discontinuities include banding, which may arise from either vertical or horizontal misalignment, or both. Mechanical devices have been described in the art that can move one or both of the print heads to correctly align them. These systems are problematic and expensive as they require mechanisms for displacing the print heads and control circuitry to run the mechanisms. Also, systems that alter the nozzle firing timing of nozzles have been developed for use with the mechanical systems to correct horizontal misalignment of two print heads.
Additionally, the droplets from the nozzles of each print head tend to be unique to that print head due to a number of variables such as differing specific resistor heating characteristics and nozzle size differences. Because ink droplet deposition is unique to each nozzle set, the change from the droplets deposited by the nozzles on one print head to those of the other print head can also degrade the quality of the resulting image. These problems are compounded where the swath height of a printer is further increased by adding more than two print heads.
A method and system are described for inkjet printing utilizing a printer having at least two print heads, by dividing an image to be imprinted upon a print medium into a plurality of raster lines with each one of the plurality of raster lines comprising a plurality of pixels. The printer then expels a plurality of ink droplets from the at least two print heads and onto the print medium corresponding to the plurality of pixels such that not all of the plurality of raster lines are formed only by one of the at least two print heads.
In another embodiment a method of depositing an image onto a print medium by a printer having at least a first print head and a second print head is disclosed, the method comprising depositing a plurality of drops of ink from the first and second print heads along a plurality of lines of print onto the print medium such that a number of the plurality of drops of ink comprising at least one of the plurality of lines of print are deposited from both the first print head and the second print head.
Another embodiment disclosed is a method of depositing an image onto a print medium by a printer having at least a first print head and a second print head comprising depositing a plurality of drops of ink from the first and second print heads along a plurality of lines of print onto the print medium such that a number of the plurality of drops of ink comprising at least one of the plurality of lines of print are deposited from both the first print head and the second print head.
In yet another embodiment, the previous embodiments a further developed by controlling a sequence of the expelling of ink droplets form each of the more than one print heads independently such that the sequence of expelling from a one of the at least two print heads may be altered relative to the others of the at least two print heads.